Wrap includes in sys/tslog.h with #ifdef TSLOG.

This is necessary because some non-kernel code #defines _KERNEL and then
includes kernel headers; as a result, it was getting conflicting versions
of curthread and curproc. Non-kernel code should probably refrain from
defining _KERNEL, but for now hiding these indirect inclusions fixes the
build.

Reported by: Michael Butler, Herbert J. Skuhra